JavaScript: .then()

The .then() method continues after a Promise succeeds and passes its result to the next step.

What you will learn

When .then() runs after a Promise fulfills
How to return a value for the next .then()
How to connect failures to .catch()

Minimal example

fetch("data.json")
  .then(response => {
    if (!response.ok) throw new Error(`HTTP ${response.status}`);
    return response.json();
  })
  .then(data => {
    console.log("Data received:", data);
  })
  .catch(error => {
    console.error("Could not load data:", error);
  });

Each callback returns a new Promise. Returning response.json() lets the next .then() wait for the parsed data. Throwing an error sends the chain to .catch().

Common mistakes

Return the next Promise or value when a later step needs it; otherwise it may receive undefined.
Do not assume fetch() rejects for HTTP errors; check response.ok.
Use .catch() to make failures visible and decide how to recover.